The present invention concerns a process for the extrusion of compositions based on alpha-olefin polymers. It concerns more particularly a process for the fabrication of objects, by extrusion followed by cooling with water, of alpha-olefin compositions, preferably based on propylene polymers.
Alpha-olefin polymers, in particular polymers of lower alpha-monoolefins such as ethylene and propylene, are thermoplastic polymers adaptable to being formed by extrusion. An important market for this transformation process resides in the fabrication of films, sheets, tubes and filaments. In the particular case of propylene polymers, a large volume of these extrudable polymers is destined for the fabrication of fibers and films. These latters can, for example, be utilized, as is, as packaging films, can be cut into strips for weaving, or can even be fibrillated for various textile uses.
Very often, the extrusion proper of the polymer is followed by an abrupt cooling treatment (hardening) of the extruded objects by means of water. Thus, extruded filaments generally pass, at the outlet of the spinneret, into a hardening tank containing water. Extruded sheets can be cooled by spraying with water after their calendering. Extruded tubes are often cooled by immersion in a water bath.
As regards films, it is known that they can be fabricated by methods called "blown film" and "flat spinneret." According to this latter, the film issuing from the spinneret is cooled by cylinders provided with internal fluid circulation ("chill roll") or by a water bath. The abrupt cooling obtained by this latter method makes it preferred for obtaining clear films. This method is very valuable for the fabrication of textiles for technical uses (weaving, rope manufacture, string or twine manufacture. . . ), for which the film must be cut into strips.
In all of the above processes, carried out continuously and implying a cooling or hardening of the extruded objects by a water bath, there is presented the problem of entrainment, by the extruded object, during the course of its advance, of more or less significant quantities of cooling water. This entrainment often becomes apparent already in connection with low extrusion speeds. The presence of this water is particularly annoying for the performance of the subsequent operations which the objects to be formed are required to undergo.
Thus, films and filaments generally undergo a stretching for the purpose of increasing their strength. Moreover, the films can be cut into strips before stretching. The presence of water disturbs this stretching and occasions frequent breaks in these filaments and strips.
Moreover, entrainment of the cooling water by extruded sheets or tubes leads to surface defects in the former and provokes an irregular shrinking and heterogeneous composition of the latter.
Numerous attempts have already been made to remedy these drawbacks. They primarily reside in a search for additives which suppress or at least reduce, the entrainment of water by objects extruded from compositions based on polyolefins, but they have never led to entirely satisfactory results. It has not been possible, in effect, to reduce in this manner the entrainment of water by objects extruded at high speed, without harming other essential properties of these objects, such as stability, coloration and anti-corrosive action.